Pb-free solder alloy

ABSTRACT

The present invention is provided to prevent the generation of whiskers via a lead (Pb)-free solder alloy. To achieve this objective, the present invention provides a Pb-free solder alloy including tin (Sn) as a first element and either boron (B) or beryllium (Be) as a second element.

TECHNICAL FIELD

The present invention relates to a solder alloy not containing lead(hereinafter, referred to as a Pb-free solder alloy), and moreparticularly, to a Pb-free solder alloy that generates no whiskers byincluding beryllium (Be) or boron (B).

BACKGROUND ART

Soldering is a technique of joining two or more members together byusing a solder having a melting point of 450° C. or less. In soldering,only the solder is melted and a base material is not melted.

A conventional solder, used in soldering, is an alloy of lead (Pb) andtin (Sn). Such Pb—Sn solders mostly comprise 63% by weight of tin andhave a eutectic composition of tin and Pb, and a melting point of 183°C., which does not thermally destroy electronic parts. In addition, thePb—Sn solders have excellent wetability for the electrodes of ball gridarrays (BGAs) or the lands of printed circuit boards (PCBs) and thusreduce the number of soldering failures.

However, when electronic apparatuses using such Pb—Sn solders aredisused, the Pb contained in these solders pollutes the environment.With the reinforcement of a restriction on the use of Pb, the Pb—Snsolders are becoming difficult to be used.

Accordingly, Pb-free solders containing no lead are recently in use. Acompound obtained by adding Ag, Cu, Zn, In, Ni, Cr, Fe, Co, Ge, P, or Gato a Sn—Ag based material, a Sn—Cu based material, a Sn—Bi basedmaterial, a Sn—Zn based material, or an alloy of each of theaforementioned materials is the main representative of Pb-free solderalloys.

A Sn—3Ag—0.5Cu compound from among Pb-free solders obtained by adding Cuto a Sn—Ag based material is good in terms of solderability, a jointstrength, and high-resistant fatigability, and is thus currently used insoldering for many electronic apparatuses. The Sn—3Ag—0.5Cu compound isalso used as a solder alloy for forming bumps and balls of BGAs.

However, when a Sn—Ag—Cu based Pb-free alloy is used as a solder for along period of time, whiskers are prone to be formed on the surface ofthe solder. The whiskers are denoted by crystals that grow from thesurface of the solder when the solder is joined with a differentmaterial and their components are diffused with each other. Thesewhiskers are sensitive to heat and moisture. When these whiskers areformed on the surface of a solder alloy, an electrical short occurswithin a circuit. Therefore, the durabilities of a BGA package and aflip-chip package are reduced.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1D are scanning electron microscope (SEM) pictures of asurface of a specimen that has just been manufactured, a surface of thespecimen subjected to a thermal shock test, a surface of the specimensubjected to a thermo-hydrostatic test, and a surface of the specimenthat was left undisturbed at a normal temperature, respectively,according to a first experiment;

FIGS. 2A through 2D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a second experiment;

FIGS. 3A through 3D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a third experiment;

FIGS. 4A through 4D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a fourth experiment;

FIGS. 5A through 5D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a fifth experiment;

FIGS. 6A through 6D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a sixth experiment;

FIGS. 7A through 7D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a seventh experiment;

FIGS. 8A through 8D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to an eighth experiment;

FIGS. 9A through 9D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a ninth experiment;

FIGS. 10A through 10D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a tenthexperiment;

FIGS. 11A through 11D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to an eleventhexperiment;

FIGS. 12A through 12D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a twelfthexperiment;

FIGS. 13A through 13D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a firstcomparative experiment;

FIGS. 14A through 14D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a secondcomparative experiment;

FIGS. 15A through 15D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a thirdcomparative experiment;

FIGS. 16A through 16D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a fourthcomparative experiment;

FIGS. 17A through 17D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a fifthcomparative experiment; and

FIGS. 18A through 18D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a sixthcomparative experiment.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

The present invention provides a Pb-free solder alloy that does notinclude lead (Pb) and also can prevent generation of whiskers.

Technical Solution

According to an aspect of the present invention, there is provided aPb-free solder alloy comprising tin (Sn) as a first element and one ofboron (B) or beryllium (Be) as a second element.

The second element of the Pb-free solder alloy may be 0.001 to 0.4% byweight of Be and the rest of the Pb-free solder alloy may be comprisedof the first element and inevitable impurities.

The second element of the Pb-free solder alloy may be 0.003 to 0.5% byweight of B and the rest of the Pb-free solder alloy may be comprised ofthe first element and inevitable impurities.

The Pb-free solder alloy may further include copper (Cu) as a thirdelement.

The third element may be 0.1 to 5.0% by weight.

The second element of the Pb-free solder alloy may be 0.001 to 0.4% byweight of Be and the rest of the Pb-free solder alloy may be comprisedof the first element, the third element, and inevitable impurities.

The second element of the Pb-free solder alloy may be 0.003 to 0.5% byweight of B and the rest of the Pb-free solder alloy may be comprised ofthe first element, the third element, and inevitable impurities.

The Pb-free solder alloy may further comprise silver (Ag) as a fourthelement.

The second element of the Pb-free solder alloy may be 0.001 to 0.4% byweight of Be and the rest of the Pb-free solder alloy may be comprisedof one of a group of the first and fourth elements and inevitableimpurities and a group of the first, third, and fourth elements andinevitable impurities.

The second element of the Pb-free solder alloy may be 0.003 to 0.5% byweight of B and the rest of the Pb-free solder alloy may be comprised ofone of a group of the first and fourth elements and inevitableimpurities and a group of the first, third, and fourth elements andinevitable impurities.

ADVANTAGEOUS EFFECTS

According to the present invention as described above, a Pb-free solderalloy capable of preventing generation of whiskers can be provided.

BEST MODE

As described above, a conventional Sn—Ag—Cu-based Pb-free solder has adisadvantage of generating whiskers on the surface thereof. However, thecause of the generation of the whiskers is not yet clearly revealed.

The inventors of the present invention paid attention to the fact thatwhen a Pb—Sn solder is bonded to a pad formed of Cu, Cu is diffusedfaster than Sn on a bonding surface between the solder and the Cu pad.

In other words, since copper (Cu) is diffused faster than tin (Sn),which is a main component of the solder, between the solder and the Cupad, the Cu is diffused in the direction of a grain boundary of thesolder. Thereafter, an intermetallic compound with a Cu6Sn5 compositionis formed in the solder.

The inventors of the present invention thought that a compressive stressapplied by the intermetallic compound to the Sn of the solder can beremoved by whiskers, which are single crystals having beard formations,growing from the surface of the solder where Sn is spread.

Accordingly, the inventors of the present invention tried to reduce thenumber of generations of a compressive stress within the Sn bypreventing intermetallic diffusion via the insertion of a metal whoseatoms are small into an interstitial site within the crystal structureof the Sn, consequently preventing the generation of whiskers.

Beryllium (Be) or boron (B) may be used as the metal with small atoms.

A Pb-free solder alloy according to the present invention is a Sn-basedmulti-element alloy that contains Sn as the main ions. Accordingly, thePb-free solder alloy according to the present invention may contain atleast 80% by weight of Sn.

As described above, the main object of the present invention is toprevent generation of whiskers within a Pb-free solder alloy.Particularly, the inventors of the present invention paid attention toBe or B as a material that can prevent the formation of a compressivestress within Sn crystals by preventing Sn and Cu from being diffusedwhen a Sn-based solder and a Cu pad are bonded together. Thus, thePb-free solder alloy according to the present invention contains Sn as afirst element and Be or B as a second element. Hence, at least 80% byweight of Sn is contained in the Pb-free solder alloy according to thepresent invention, and thus the Pb-free solder alloy according to thepresent invention is referred to as a Sn-based alloy.

The Pb-free solder alloy according to the present invention may contain0.001 to 0.4% by weight of Be or 0.003 to 0.5% by weight of B.

In this case, a sufficient amount of Be or B, as the second element, isinserted into an interstitial site in the Sn, which is the firstelement, as compared with a case where the Pb-free solder alloyaccording to the present invention contains less than 0.001% by weightof Be or less than 0.003% by weight of B. Thus, as described above, theeffect of preventing growth of an intermetallic compound between Sn andCu is high, and as described later, whiskers may not be generated evenunder harsh conditions such as a thermal shock test, athermo-hydrostatic test, etc. Also, when the Pb-free solder alloyaccording to the present invention contains more than 0.4% by weight ofBe or more than 0.5% by weight of B, the Be or B inserted into theinterstitial site in the Sn is saturated, thereby causing an increase inthe manufacturing costs and a degradation of economical efficiency.

The Pb-free solder alloy may further contain Cu as a third element. Inthis case, 0.1 to 5.0% by weight of Cu may be included. Thus, themechanical strength of the Pb-free solder alloy may increase, ascompared with when the Cu content is less than 0.1% by weight, and thewetability thereof may improve, as compared with when the Cu contentexceeds 5.0% by weight.

The Pb-free solder alloy may further include silver (Ag) as a fourthelement. Here, 1.0 to 3.0% by weight of Ag may be included. In thiscase, the thermal shock tolerance of the Pb-free solder alloy maysignificantly increase, as compared with when the Ag content is lessthan 1.0% by weight, and the drop tolerance thereof may improve, ascompared with when the Ag content exceeds 3.0% by weight.

Such a Pb-free solder may be manufactured in various forms, such as, aball, a cream, a bar, a wire, etc.

MODE OF THE INVENTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown. The following experiments are not to be construedas limiting the invention but are described to provide a thoroughunderstanding of the present invention.

FIRST EMBODIMENT

A Pb-free solder alloy, according to the first embodiment, is a Sn—Be—Cuternary alloy.

In the first embodiment, a Be—Cu alloy was first manufactured, Sn wasmelted in a melting pot, and the Be—Cu alloy was melted in the meltingpot, thereby producing a melt. After the temperature of the melt waskept for a certain period of time between 600° C. and 650° C., the meltwas tapped from the melting pot and cast into a bar-shaped Sn—Be—Cusolder alloy specimen.

After the surface of a JIS 2 type Cu base with a comb shape waspolished, a flux EC-19S-8 by Tamura-Kaken Corporation was coated on thepolished surface of the Cu base. Thereafter, the prepared Sn—Be—Cusolder alloy specimen was melted in a fused silica tube by apredetermined amount, and the Cu base was digested in the resultantfused silica tube for 3 seconds so as to perform dip soldering. Next,the dip-soldered substrate was dipped in ethyl acetate, and thenresidues of the flux were removed through ultrasonic cleaning, therebymanufacturing experimental specimens.

The following Table 1 shows the contents of Sn, Be, and Cu in theexperimental specimens manufactured according to the first embodiment.The unit of the numbers shown in Table 1 is % by weight, and the numbersare the contents of the elements inserted into the melt. Besides theelements stated in Table 1, very small amounts of impurities, such asphosphorus (P), nickel (Ni), and cobalt (Co), may be further included inthe melt.

In Table 1, the column “right after the manufacture” indicates whetherwhiskers were generated on the experimental specimens just after beingmanufactured, the column “thermal shock” indicates whether whiskers weregenerated on the surfaces of the manufactured experimental specimenswhich underwent thermal shock tests in which a specimen is maintainedbetween −55° C. and 80° C. 1000 times for 20 minutes per one time, thecolumn “thermo-hydrostatic test” indicates whether whiskers weregenerated on the surfaces of the manufactured experimental specimenswhich underwent thermo-hydrostatic tests in which a specimen ismaintained for 1000 hours at a humidity of 90% and a temperature of 80°C., and the column “leaving undisturbed at a normal temperature”indicates whether whiskers were generated on the surfaces of themanufactured experimental specimens which were left for 12 months at anormal temperature. “Undetected”, as shown in the below tables includingTable 1, indicates that no whiskers were generated in the manufacturedexperimental specimens, and “Detected” indicates that whiskers weregenerated in the manufactured experimental specimens.

TABLE 1 leaving undisturbed Right after Thermal Thermo-hydrostatic atnormal Experiment Sn Be Cu manufacture shock test temperature Firstexperiment 99.9833 0.0005 0.0162 Undetected Detected Detected DetectedSecond 99.967 0.001 0.032 Undetected Undetected Undetected Undetectedexperiment Third experiment 99.484 0.020 0.496 Undetected UndetectedUndetected Undetected Fourth experiment 94.804 0.200 4.996 UndetectedUndetected Undetected Undetected Fifth experiment 94.604 0.400 4.996Undetected Undetected Undetected Undetected

FIGS. 1A through 1D are scanning electron microscope (SEM) pictures of asurface of a specimen that has just been manufactured, a surface of thespecimen subjected to a thermal shock test, a surface of the specimensubjected to a thermo-hydrostatic test, and a surface of the specimenthat was left undisturbed at a normal temperature, respectively,according to a first experiment.

FIGS. 2A through 2D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a second experiment.

FIGS. 3A through 3D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a third experiment.

FIGS. 4A through 4D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a fourth experiment.

FIGS. 5A through 5D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a fifth experiment.

As can be seen in Table 1 and FIGS. 1A through 5D, no whiskers weregenerated on a surface of the Sn—Be—Cu ternary alloy according to thefirst embodiment just after being manufactured. However, in the firstexperiment where the content of beryllium (Be) is less than 0.001% byweight, whiskers were generated on the surface of the Sn—Be—Cu ternaryalloy that underwent a thermal shock test, the surface thereof thatunderwent a thermo-hydrostatic test, and the surface thereof that wasleft undisturbed at a normal temperature.

In the first experiment, the detected whiskers have lengths of 3.4 μm onaverage, and the number of whiskers per unit area (mm²) is 3.

Although whiskers were generated after harsh conditions in the firstexperiment, the lengths of the whiskers are significantly less thanthose in comparative experiments that are to be described later, and thenumber of whiskers per unit area is small. Accordingly, the Sn—Be—Cuternary alloy according to the first embodiment provides good effectscompared with conventional ones.

In Table 1, no whiskers were detected in the second through fifthexperiments where the content of Be is at least 0.001% by weight. Thus,a Sn—Be—Cu ternary alloy including at least 0.001% by weight of Be ispreferable.

SECOND EMBODIMENT

A Pb-free solder alloy according to the second embodiment is aSn—Be—Cu—Ag quaternary alloy.

In the second embodiment, a Be—Cu alloy was first manufactured, Sn wasmelted in a melting pot, and the Be—Cu alloy and silver (Ag) were meltedin the melting pot, thereby producing a melt. After the temperature ofthe melt was kept for a certain period of time between a temperature of600° C. to 650° C., the melt was tapped from the melting pot and castinto a bar-shaped Sn—Be—Cu—Ag solder alloy specimen.

The bar-shaped Sn—Be—Cu—Ag solder alloy specimen was processed as in thefirst embodiment so as to manufacture experimental specimens.

The following Table 2 shows the contents of Sn, Be, Cu, and Ag in theexperimental specimens manufactured according to the second embodiment.The unit of the numbers shown in Table 2 is % by weight, and the numbersare the contents of the elements inserted into the melt. Besides theelements stated in Table 2, very small amounts of impurities, such as P,Ni, and Co, may be further included in the melt.

Table 2 also indicates whether whiskers were generated on the surfacesof the manufactured experimental specimens right after beingmanufactured, after a thermal shock test, after a thermo-hydrostatictest, and after being left undisturbed at a normal temperature, underthe same conditions as those in Table 1.

TABLE 2 leaving undisturbed at Right after Thermal Thermo-hydrostaticnormal Experiment Sn Ag Cu Be manufacture shock test temperature Sixth98.900 1.000 0.097 0.003 Undetected Undetected Undetected Undetectedexperiment Seventh 98.300 1.000 0.679 0.021 Undetected UndetectedUndetected Undetected experiment Eighth 96.900 3.000 0.097 0.003Undetected Undetected Undetected Undetected experiment Ninth 94.00 3.002.88 0.12 Undetected Undetected Undetected Undetected experiment

FIGS. 6A through 6D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a sixth experiment.

FIGS. 7A through 7D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a seventh experiment;

FIGS. 8A through 8D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to an eighth experiment.

FIGS. 9A through 9D are SEM pictures of a surface of a specimen that hasjust been manufactured, a surface of the specimen subjected to a thermalshock test, a surface of the specimen subjected to a thermo-hydrostatictest, and a surface of the specimen that was left undisturbed at anormal temperature, respectively, under the same conditions as those ofthe first experiment, according to a ninth experiment.

As can be seen in Table 2 and FIGS. 6A through 9D, no whiskers weregenerated on a surface of the Sn—Be—Cu ternary alloy according to thesecond embodiment just after being manufactured, a surface thereof thatunderwent a thermal shock test, a surface thereof that underwent athermo-hydrostatic test, and a surface thereof that was left undisturbedat a normal temperature.

THIRD EMBODIMENT

A Pb-free solder alloy according to the third embodiment is a Sn—B—Cuternary alloy.

In the third embodiment, Sn was melted in a melting pot, and boron (B)and copper (Cu) were then melted in the resultant melting pot, therebyproducing a melt. After the temperature of the melt was kept for acertain period of time between a temperature of 600° C. to 650° C., themelt was tapped from the melting pot and cast into a bar-shaped Sn—B—Cusolder alloy specimen.

The bar-shaped Sn—B—Cu solder alloy specimen was processed as in thefirst embodiment so as to manufacture experimental specimens.

The following Table 3 shows the contents of Sn, B, and Cu in theexperimental specimens manufactured according to the third embodiment.The unit of the numbers shown in Table 3 is % by weight, and the numbersare the contents of the elements inserted into the melt. Besides theelements stated in Table 3, very small amounts of impurities, such as P,Ni, and Co, may be further included in the melt.

Table 3 also indicates whether whiskers were generated on the surfacesof the manufactured experimental specimens right after beingmanufactured, after a thermal shock test, after a thermo-hydrostatictest, and after being left undisturbed at a normal temperature, underthe same conditions as those in Table 1.

TABLE 3 leaving undisturbed Right after Thermal Thermo-hydrostatic atnormal Experiment Sn B Cu manufacture shock test temperature Tenth99.989 0.001 0.010 Undetected Detected Detected Detected experimentEleventh 99.987 0.003 0.010 Undetected Undetected Undetected Undetectedexperiment Twelfth 98.5 0.5 1.0 Undetected Undetected UndetectedUndetected experiment

FIGS. 10A through 10D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a tenthexperiment.

FIGS. 11A through 11D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to an eleventhexperiment.

FIGS. 12A through 12D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a twelfthexperiment.

As can be seen in Table 3 and FIGS. 10A through 12D, no whiskers weregenerated on a surface of the Sn—B—Cu ternary alloy according to thethird embodiment just after being manufactured. However, in the tenthexperiment where the content of B is less than 0.003% by weight,whiskers were generated on the surface of the Sn—B—Cu ternary alloy thatunderwent a thermal shock test, the surface thereof that underwent athermo-hydrostatic test, and the surface thereof that was leftundisturbed at a normal temperature.

In the tenth experiment, the detected whiskers have lengths of 3.0 μm onaverage, and the number of whiskers per unit area (mm²) is 5.

Although whiskers were generated after harsh conditions in the tenthexperiment, the lengths of the whiskers are significantly less thanthose in comparative experiments that are to be described later, and thenumber of whiskers per unit area is small. Accordingly, the Sn—Be—Cuternary alloy according to the third embodiment provides good effectscompared with conventional ones.

In Table 3, no whiskers were detected in the eleventh and twelfthexperiments where the content of B is at least 0.003% by weight. Thus, aSn—Be—Cu ternary alloy including at least 0.003% by weight of B ispreferable.

(Comparative Experiments)

Pb-free solder alloys according to the comparative experiments are aSn—Cu binary alloy and a Sn—Ag—Cu ternary alloy. A Sn—Cu ingot and aSn—Ag—Cu ingot by Samhwa Non-ferrous Metal Ind. Co., Ltd were used inthe comparative experiments. Experimental specimens were manufacturedusing the Sn—Cu ingot and the Sn—Ag—Cu ingot according to the samemethod as in the first embodiment. The unit of the contents shown inTable 4 is % by weight.

Table 4 also indicates whether whiskers were generated on the surfacesof the manufactured experimental specimens right after beingmanufactured, after a thermal shock test, after a thermo-hydrostatictest, and after being left undisturbed at a normal temperature, underthe same conditions as those in Tables 1 through 3.

TABLE 4 Leaving undisturbed at Comparative Right after ThermalThermo-hydrostatic normal experiment Sn Ag Cu manufacture shock testtemperature First comparative 99.9 0.0 0.1 Undetected Detected DetectedDetected experiment Second comparative 99.3 0.0 0.7 Undetected DetectedDetected Detected experiment Third comparative 95.0 0.0 5.0 UndetectedDetected Detected Detected experiment Fourth comparative 98.9 1.0 0.5Undetected Detected Detected Detected experiment Fifth comparative 98.03.0 0.5 Undetected Detected Detected Detected experiment Sixthcomparative 94.0 3.0 1.0 Undetected Detected Detected Detectedexperiment

FIGS. 13A through 13D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a firstcomparative experiment.

FIGS. 14A through 14D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a secondcomparative experiment.

FIGS. 15A through 15D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a thirdcomparative experiment.

FIGS. 16A through 16D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a fourthcomparative experiment.

FIGS. 17A through 17D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a fifthcomparative experiment.

FIGS. 18A through 18D are SEM pictures of a surface of a specimen thathas just been manufactured, a surface of the specimen subjected to athermal shock test, a surface of the specimen subjected to athermo-hydrostatic test, and a surface of the specimen that was leftundisturbed at a normal temperature, respectively, under the sameconditions as those of the first experiment, according to a sixthcomparative experiment.

As can be seen from Table 4 and FIGS. 13A through 18D, whiskers weregenerated on the surfaces of all of the Sn-based solder alloys includingneither Be nor B.

In the first and tenth experiments and the first through sixthcomparative experiments, whiskers were generated on the surface of themanufactured specimen that underwent a thermal shock test, the surfacethereof that underwent a thermo-hydrostatic test, and the surfacethereof that was left undisturbed at a normal temperature. Table 5 showsthe mean of the lengths of the generated whiskers and the number ofwhiskers per unit area.

TABLE 5 Average Number of whisker whiskers per length unit area Firstexperiment  3.4 μm  3/mm² Tenth experiment  3.0 μm  5/mm² First throughthird 14.4 μm 11/mm² comparative experiments Fourth through sixth 11.8μm 14/mm² comparative experiments

As can be seen from Table 5, the solder alloys of the first and tenthexperiments have whiskers that are significantly short and the number ofwhich is significantly small, as compared with the Sn—Cu solder alloysof the first through third comparative experiments and the Sn—Ag—Cusolder alloys of the fourth through sixth comparative experiments.

Accordingly, even when an extremely small amount of Be, namely, lessthan 0.001% by weight of Be, is added to Sn or even when an extremelysmall amount of B, namely, less than 0.003% by weight of B, is added toSn, an effect of preventing the generation of whiskers is significantlyhigh, as compared with comparative examples in which neither Be nor B isadded.

As described above, a solder alloy according to the present inventioncan be prevented from having whiskers even when being under badconditions.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby one of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

The solder alloy according to the present invention can be used insoldering wires of various machines and electronic apparatuses.

1. A Pb-free solder alloy comprising: tin (Sn) as a first element; andone of boron (B) or beryllium (Be) as a second element.
 2. The Pb-freesolder alloy of claim 1 , wherein the second element of the Pb-freesolder alloy is 0.001 to 0.4% by weight of Be and the rest of thePb-free solder alloy is comprised of the first element and inevitableimpurities.
 3. The Pb-free solder alloy of claim 1 , wherein the secondelement of the Pb-free solder alloy is 0.003 to 0.5% by weight of B andthe rest of the Pb-free solder alloy is comprised of the first elementand inevitable impurities.
 4. The Pb-free solder alloy of claim 1 ,further comprising copper (Cu) as a third element.
 5. The Pb-free solderalloy of claim 4, wherein the third element is 0.1 to 5.0% by weight. 6.The Pb-free solder alloy of claim 4, wherein the second element of thePb-free solder alloy is 0.001 to 0.4% by weight of Be and the rest ofthe Pb-free solder alloy is comprised of the first element, the thirdelement, and inevitable impurities.
 7. The Pb-free solder alloy of claim4, wherein the second element of the Pb-free solder alloy is 0.003 to0.5% by weight of B and the rest of the Pb-free solder alloy iscomprised of the first element, the third element, and inevitableimpurities.
 8. The Pb-free solder alloy of claim 1, further comprisingsilver (Ag) as a fourth element.
 9. The Pb-free solder alloy of claim 8,wherein the second element of the Pb-free solder alloy is 0.001 to 0.4%by weight of Be and the rest of the Pb-free solder alloy is comprised ofone of a group of the first and fourth elements and inevitableimpurities and a group of the first, third, and fourth elements andinevitable impurities.
 10. The Pb-free solder alloy of claim 8, whereinthe second element of the Pb-free solder alloy is 0.003 to 0.5% byweight of B and the rest of the Pb-free solder alloy is comprised of oneof a group of the first and fourth elements and inevitable impuritiesand a group of the first, third, and fourth elements and inevitableimpurities.
 11. The Pb-free solder alloy of claim 5, wherein the secondelement of the Pb-free solder alloy is 0.001 to 0.4% by weight of Be andthe rest of the Pb-free solder alloy is comprised of the first element,the third element, and inevitable impurities.
 12. The Pb-free solderalloy of claim 5, wherein the second element of the Pb-free solder alloyis 0.003 to 0.5% by weight of B and the rest of the Pb-free solder alloyis comprised of the first element, the third element, and inevitableimpurities.
 13. The Pb-free solder alloy of claim 4, further comprisingsilver (Ag) as a fourth element.
 14. The Pb-free solder alloy of claim13, wherein the second element of the Pb-free solder alloy is 0.001 to0.4% by weight of Be and the rest of the Pb-free solder alloy iscomprised of one of a group of the first and fourth elements andinevitable impurities and a group of the first, third, and fourthelements and inevitable impurities.
 15. The Pb-free solder alloy ofclaim 13, wherein the second element of the Pb-free solder alloy is0.003 to 0.5% by weight of B and the rest of the Pb-free solder alloy iscomprised of one of a group of the first and fourth elements andinevitable impurities and a group of the first, third, and fourthelements and inevitable impurities.